Severe combined immunodeficiency disease (SCID) is a fatal congenital disease of children. Approximately 1/3 of cases are associated with the deficiency of adenosine deaminase (ADA) and some of the remaining cases with the X-linked variant have mutations of the IL-2 gamma receptor gene. The severe nature of this illness, the deficiency of single peptide proteins, and the ability to correct the disease by allogeneic bone marrow transplantation have led many investigators to explore the use of gene transfer technology as a therapeutic option for the treatment of SCID. The long term goal of the work proposed here is to utilize gene transfer technology to effect efficient transfer into reconstituting hematopoietic stem cells and stable expression in progeny of these cells of genetic sequence defective in SCID in order to provide life-long cure of this disease. The basis of the approaches to gene transfer proposed in this grant include the realization that published data, including from our own laboratory, demonstrate that current protocols for infection of reconstituting stem cells of large animals species are not optimal. Furthermore, a wealth of experimental data suggests that the hematopoietic microenvironment is a source of both positive and negative regulators of hematopoiesis and these signals may be important for successful and efficient transduction of primitive hematopoietic stem cells. The hypothesis to be tested in this proposal is that transduction of long term reconstituting stem cells can be accomplished in large animal transplant models by retroviral infection in the presence of critical components of the hematopoietic microenvironment. We will utilize our previously characterized simplified retroviral vector which expresses the ADA cDNA and both in vitro and in vivo assays for human stem cells to analyze transduction efficiency and expression of introduced sequences. In addition, we will continue to use primate transplants to evaluate gene transfer technology in a large animal autologous transplant model.